1. Field of the Invention
The present invention relates to a ball screw for transforming rotational motion into linear motion or transforming linear motion into rotational motion.
2. Description of the Related Art
A ball screw includes: a screw shaft having a spiral ball rolling groove formed in its outer peripheral surface; a ball nut having a spiral ball rolling groove formed in its inner peripheral surface in accordance with the spiral ball rolling groove of the screw shaft; and a large number of balls disposed in a spiral space formed by the two spiral ball rolling grooves so as to be able to roll.
When the screw shaft and the ball nut are rotated relatively to each other, the screw shaft and the ball nut move relatively to each other spirally, smoothly and accurately through rolling of the large number of balls while the balls circulate in a ball circulation path provided in the ball nut.
The ball rolling grooves are generally formed by thread grinding finish. The thread grinding process has a tendency to increase periodic machining error (undulation) in a finished surface due to whirling of a grindstone compared with a cylindrical grinding or annular grooving process.
When the distance between undulation peaks on each ball rolling groove 1 does not coincide with the distance between the load balls 2 as shown in FIG. 5A, the influence of machining error on relative motion can be however suppressed by an error averaging effect regardless of more or less machining error on each ball rolling groove 1 because the large number of load balls 2 bear the load imposed on the ball screw.
When the distance between undulation peaks on each ball rolling groove 1 coincides with the distance between the load balls 2 (or a value obtained by dividing the distance by an integer) as shown in FIG. 5B, the averaging effect is however spoiled so that the relative motion is directly influenced by more or less machining error (undulation) on the ball rolling groove 1. Hence, there is a problem that the machining error may cause incidence of vibration or noise.
It is therefore necessary to finish the ball rolling groove 1 in the ball screw accurately to make machining error (undulation) sufficiently small. There is however a problem that this may have large influence on productivity and production cost.